This invention relates to a two-color copying apparatus suitable for use as office equipment for producing from a document or picture in two colors, including black and red or other chromatic color, a copy or copies of the document or picture in the same colors.
When it is desired to produce a copy of a painting or picture in color by using a color copying apparatus, it is necessary that the copy or copies produced have the same color tone and contrast as the original. In this case, beams of light obtained by passing an optical image of the original through a three color separators of blue, green and red color separators are used to successively expose one and the same photosensitive member thereto to obtain three discrete latent images which are successively developed into visible images by using three different kinds of toners or toners of yellow, magenta and cyan in color. The visible images obtained in this way are successively printed in overlapping relation by transfer printing in a transfer-printing sheet which is wound on a transfer printing drum to obtain a copy of the original in original colors.
Some disadvantages are associated with this type of color copying apparatus. Difficulties would be experienced in achieving a delicate balance in the density of toner images of different colors, and highly advanced techniques would be required for precisely and accurately superposing the toner images one over another on a transfer-printing drum. Thus, it would require operators of high skills to perform the operation of producing the desired copy or copies in the same color as the original and the operation itself would be time-consuming and high in cost.
Meanwhile, documents handled in offices, such as memorandums on business transactions or reports on research are generally in black ink. However, it sometimes happens that lines are drawn in red beneath some letters or sentences or around some drawing figures to draw attention thereto. If it is possible to produce a copy or copies of such documents in two colors or black and red, for example, the trouble of coloring a monochrome copy could be eliminated. In this case, it is not necessary that the color tone and contrast obtained in the color copy be exactly the same as in the original as is the case with copying a painting or color picture as described hereinabove, and the image in red need not necessarily be high in resolution.
To this end, proposals have been made to use a two-color copying apparatus of the type which comprises a half reflecting mirror located posterior to an image forming lens of an original exposing and scanning optical system to split a beam of light reflected by an original into a sub-beam reflected by the half reflecting mirror and a sub-beam transmitted through the half reflecting mirror. The sub-beams of light obtained by using the half reflecting mirror are projected on to surfaces of separate photosensitive members to expose them to form on each photosensitive member by a predetermined process an electrostatic latent image having image regions corresponding to regions of the original which are black in color and image regions corresponding to regions of the original which are red or any other chromatic color. The black image regions of each electrostatic latent image are developed by using a black toner and the image regions thereof of other color than black are developed by using a toner of that color, and the toner images formed on the two photosensitive members are successively printed by transfer-printing in overlapping relation on transfer-printing sheets successively fed to a developing station while they are kept in contact with the surfaces of the photosensitive members.
In an original image forming optical system using mirrors, the resolution of an image forming surface may vary depending on the flatness of the surface of the mirrors. Particularly when the mirror is located in the vicinity of the lens of the optical system, the resolution is greatly affected by the flatness of the surface of the mirror. The half reflecting mirror described hereinabove is located immediately posterior to the image forming lens. Thus, if the half reflecting mirror were low in flatness, the image forming surface would have reduced resolution with respect to the sub-beam of light reflected by the half reflecting mirror. A reduction in resolution would be marked when it is desired to obtain a copy in a smaller size than the original.
Meanwhile, the sub-beam of light transmitted through the half reflecting mirror is impervious to influences exerted by a medium (glass of the half reflecting mirror), so that the use of the half reflecting mirror causes no appreciable reduction in the resolution of the image forming surface.
In the two-color copying apparatus of the construction referred to hereinabove, the toner images are printed by transfer-printing from the photosensitive members on transfer-printing sheets which are successively fed to the photosensitive members in such a manner that the image regions of black color and chromatic color are disposed on each copy produced in the same positional relation as in the original. Consequently, the two photosensitive members are spaced apart from each other a small distance, and difficulties might be experienced in arranging various units of the electrophotographic copying apparatus around the two photosensitive members depending on the positions in which the two sub-beams of light are projected on to the surfaces of the photosensitive members to form electrostatic latent images thereon.